The alimentation of hospitalized infants is still unsatisfactory and, at times, life-threatening. This research proposal is designed to seek answers to fundamental nutrition problems by considering the intraluminal influences of substrate sugars, pentagastrin, pancreatic enzymes, and bile salts on jejunal enterocytes from fetal, chow fed adult, and starved adult rabbits. These luminal agents will be added to both jejunal organ and tissue culture media to evaluate their inductive effect on sucrase-isomaltase (SI) activity. Simultaneous addition of 3H-thymidine to media will label proliferating enterocytes permitting evaluation of migration of enterocytes up the villus in organ culture. Horizontal cryostat tissue planing of organ cultures over a 48 hour period will localize the enzyme induction site to either villus and/or crypt cells. To eliminate mesenchymal factors, a method will be developed to establish crypt cells in tissue culture. Where and when pre-SI antigen appears will be evaluated in fetal rabbits by indirect fluorescent antibody against SI and radioimmunoassay against SI antigen-antibody complex. Whether substrate induction involves new enzyme formation will be examined by labeled glucosamine activity in brush border preparations applied to polyacrylamide gel electrophoresis for enzyme localization. Whether inductive monosaccharides are directly or indirectly involved in enzyme synthesis will be evaluated with tagged fructose and glucose utilizing ultrastructural radioautographic techniques. Results from this proposal may provide a model in-vitro system to reliably test hypotheses and establish basic information to derive adequate safe in-vivo feeding practices for various disease states.